THE CHORIONIC VILLI. 367 



Our knowledge of the human trophoderm being still very imperfect, its full 

 history is partly a matter of supposition. The appearances described indicate 

 that the trophoderm undergoes a rapid degeneration, during which the cells fuse, 

 while their protoplasm becomes a hyaline material. - We must then further suppose 

 that the degenerated substance is resorbed and disappears altogether. Finally, 

 we must assume that the entire trophoderm does not disappear, but that enough 

 is preserved to form the permanent covering of the villi. 



It may be noted that the specimen on which the above description is based 

 agrees essentially with the specimen described by Siegenbeek van Heukelom, which 

 is regarded as normal. 



The Chorionic Villi. 



The villi may be obtained in connebtion with the preparations of the uterus 

 and placenta. In order to see the youngest stages of the first villi it "is necessary 

 to have the chorionic membrane of the second or early part of the third week. At 

 this stage the trophoderm is present and the first villi are appearing (compare page 

 115). To study the growth and form of the villi, single villi or pieces of villi 

 should be snipped off from the chorion at various stages. Such pieces may. be 

 examined as opaque objects in alcohol, or they may be stained and mounted as 

 permanent preparations. To obtain injected villi it is best to inject the placenta 

 through one of the arteries of the umbilical cord, using as the injecting mass 

 gelatin colored with carmine or Prussian blue. Such injections are very easily 

 made. 



Branching of the Villi. The formation of a branch is usually initiated by an- 

 outgrowth of the ectoderm. Branches grow very rapidly; the outgrowth which 

 forms the branch occurs with every degree of participation of the mesoderm. 

 The two extremes are, first, the bud consisting wholly of epithelium, which may 

 become a process with a long, thin pedicle and a thickened free end remaining 

 sometimes entirely without mesoderm; later the mesoderm penetrates it and 

 completes the structure. Second, a thick bud with a well-developed cord of con- 

 nective tis'sue and having a nearly cylindrical form. Between these extremes every 

 intermediate stage can be found. The tips of the branches are for the most part 

 free, but some of them come in contact with the surfaces of the decidua and 

 penetrate it for a short distance. By this means the villi of the embryo are 

 attached to the decidua of the mother. The villi do not penetrate the glands 

 of the uterus at any period, as was at one time supposed. The ectoderm on the 

 tip of the villi, where it is in contact with decidual tissue, undergoes a hyaline 

 degeneration. 



The shape of the villi varies according to the part of the chorion and the age 

 of the embryo. Over the chorion laeve there is first an arrest of development and 

 a subsequent slow degeneration of the tissues which lose all recognizable organ- 

 ization of the protoplasm, and to a large extent of their nuclei also. At the same 



